Many aquatic animals are highly sensitive to certain xenobiotic compounds. Such organisms are used as models to identify those physiological processes most sensitive to environmental pollutants. Particular emphasis is placed on membrane functions which underlie these processes. We have utilized as a model system the blue crab (Callinectes sapidus) which is extremely sensitive to organochlorine pesticides, particularly at low environmental salinity where its survival depends on its ability to osmoregulate. The objective is to establish the mechanism(s) responsible for the sensitivity of crustacea to organochlorine compounds. Parameters examined include 1) the levels of Na,K-ATPase in the gills after adaptation to various environmental salinities; 2) the ouabain binding characteristics of the enzyme; 3) the activity of Na,K-ATPase enzyme after administration of DDT both in vitro and in vivo; 4) the mode of inhibition of the enzyme in vitro; and 5) the ability of the intact animal to maintain hemolymph osmolality and ion levels after intravascular administration of DDT.